An ISIS memo from late 2015 reveals the terrorist network, which runs a well-oiled caliphate in Syria and Iraq, is reducing its fighters' salaries — which range on average from $400 to $1,200 per month — by half.

The memo was obtained by Aymenn Jawad al-Tamimi, a Middle East Forum Jihad Intel research fellow. Al-Tamimi has established sources within al-Raqqa who have been leaking documents to the jihad specialist, CNNMoney reported.

He has become a trusted authority on ISIS, even appearing before the United Kingdom's House of Commons Defense Committee to offer his analysis of Iraq and ISIS in 2014.

"So on account of the exceptional circumstances the Islamic State is facing, it has been decided to reduce the salaries that are paid to all mujahideen by half, and it is not allowed for anyone to be exempted from this decision, whatever his position," the ISIS memorandum read, as translated on al-Tamimi's website.

"Let it be known that work will continue to distribute provisions twice every month as usual," the memo concluded.

ISIS makes its money primarily from captured oil rigs and taxing people who reside within the boundaries of its caliphate, according to CNNMoney. But in recent months, ISIS has progressively suffered financial loss as a result of the United States military's repeated attacks, which aim to financially paralyze the organization.

A series of attacks on ISIS' oil operations in late 2015, for example, dramatically reduced its profitability, CNNMoney reported, from "$40 million every month" to "only a fraction of that."

Then, on Jan. 10, the U.S. took a more direct approach by bombing a building in which ISIS was reportedly storing a vast amount of money.

"We're already seeing a very significant slowdown in the operation," a senior State Department official said in a special briefing on Dec. 4.

"But there is still a functioning — relatively speaking, functioning — capability, but without a doubt, it's under strain. And it's much more difficult. And you've seen that the hours of power generation have been cut down," the official added.